JP5020526B2 - Alloyed hot-dip galvanized steel sheet with excellent corrosion resistance, workability, and paintability and method for producing the same - Google Patents

Description

  The present invention relates to an alloyed hot-dip galvanized steel sheet excellent in corrosion resistance, workability, and paintability using an ultra-low carbon steel sheet as a base plate and a method for producing the same.

Conventionally, alloyed hot-dip galvanized steel sheets have been used mainly as automotive or architectural steel sheets because of their excellent coating film adhesion and corrosion resistance after painting. Therefore, alloyed hot-dip galvanized steel sheets made from ultra-low carbon steel sheets are often used.
However, alloyed hot-dip galvanized steel sheets based on these ultra-low carbon steel sheets have problems that the corrosion resistance of bare steel and the corrosion resistance of scratched parts are not always sufficient, and further, powdering is suppressed during processing. There is also a problem that it is difficult to suppress both flaking and flaking, and a problem that appearance defects tend to occur during electrodeposition coating.

For example, Patent Document 1 has excellent corrosion resistance including a Zn—Fe alloy layer as a first layer on a steel plate, Fe: 8 to 15%, Ni: 0.1 to 2%, and Al of 1% or less as a second layer. An alloyed hot-dip galvanized steel sheet is disclosed.
Patent Document 2 discloses that a Zn plating bath containing Al: 0.05 to 0.25% is rapidly heated to 430 to 500 ° C. after 0.2 to ² g / m ² of Ni is pre-plated on the steel sheet surface. A method for producing an alloyed hot-dip galvanized steel sheet with excellent corrosion resistance is disclosed, which comprises hot-dip plating and performing alloying heat treatment at 470 to 550 ° C. for 10 to 40 seconds immediately above wiping.

  However, what is substantially disclosed in these documents is a hot-rolled low-carbon Al-killed steel sheet, and no knowledge about the ultra-low-carbon steel sheet intended by the present invention is shown. Compared with low-carbon steel sheets, ultra-low-carbon steel sheets have higher ferrite grain boundary cleanliness, alloying progresses unevenly, and Γ layers tend to grow. I can't do it. In addition, Patent Documents 1 and 2 do not show any knowledge about workability and paintability.

In Patent Document 3, Fe: 8 to 13% is obtained by hot dipping and alloying in a bath containing less than 0.2% Al and 0.01 to 0.5% Ni in a plating bath. An alloyed hot-dip galvanized steel sheet containing Al: less than 0.5%, Ni: 0.02 to 1%, and the balance Zn, and having a Γ layer thickness of the ground iron interface of 0.5 μm or less is disclosed.
However, even in this document, what is substantially disclosed is a low-carbon steel sheet, and there is no knowledge about the ultra-low-carbon steel sheet intended by the present invention. Even if it is applied to the above, it is practically impossible to make the Γ layer thickness 0.5 μm or less, and the corrosion resistance, workability, and paintability intended by the present invention are quite insufficient.

Patent Document 4 discloses a method for producing an alloyed hot-dip galvanized steel sheet that is annealed, hot-dip galvanized, and alloyed after an extremely low carbon steel sheet is plated with ²⁰ to 70 mg / m ² of Ni. . However, this method does not have an effect of improving the corrosion resistance, and the processability is not sufficient.
In Patent Document 5, plating is performed with a hot dip galvanizing bath containing Al: 0.1 to 0.2% and Ni: 0.04 to 0.2%, and the temperature rising rate is 10 to 20 ° C./s. An alloyed hot-dip galvanized steel sheet excellent in slidability and paintability, characterized by being alloyed with 1 to 40% of the surface and coated with 1 to 10 μ ζ layer, is disclosed. However, in this case, there is a drawback that processability, particularly powdering property and corrosion resistance are not sufficient.

In Patent Document 6, Ni is added to a hot dip galvanizing bath containing Al, and further plating is performed in a bath containing at least one of Pb, Sb, Bi, and Sn, and alloying is performed under predetermined conditions. Al: 0.1 to 0.25%, Fe: 6 to 18%, Ni: 0.05 to 0.3%, Pb, Sb, Bi, Sn containing 0.001 to 0.01% An alloyed hot dip galvanized steel sheet is disclosed.
However, in this case, the bath becomes a quaternary system, and management becomes complicated, and dross in which Ni and Al are combined in the bath is likely to occur, and if this is caught in the plating layer, it is a cause of deterioration in corrosion resistance. Therefore, it is not preferable.

  Further, the ultra-low carbon steel sheet to which Ti is added has a feature that extremely excellent deep drawability and ductility can be stably obtained in a wide component range. However, when this steel sheet is hot dip galvanized and further alloyed, the grain boundaries are cleaned by the effect of Ti in the steel, so the alloying reaction is accelerated at the grain boundaries, resulting in outburst. The reaction is likely to occur, the overalloy is likely to proceed, and the powdering property deteriorates.

In order to solve this problem, a method for producing an alloyed hot-dip galvanized steel sheet in which powdering properties are improved by adding Nb together with Ti and controlling the alloying reaction occurring at the grain boundaries is disclosed. (For example, refer to Patent Documents 7 to 10). However, since these are those in which Nb is further added to Ti, the addition cost of Nb is high, which is not very economical.
On the other hand, as a technique for improving the powdering property of a Ti-added ultra-low carbon steel sheet without adding Nb in combination, Patent Document 11 describes a grain boundary by controlling a water vapor atmosphere in a cooling process after recrystallization annealing. A method is disclosed in which the outburst during the alloying reaction is suppressed by oxidizing. However, this method not only makes it difficult to control oxidation, but also tends to adversely affect the plating appearance.

Furthermore, Patent Document 12 discloses a method in which the Al concentration in the hot dipping bath is 0.12 to 0.2% higher than usual and a phase having a high Al concentration is localized at the base metal-plating interface. However, in this case, the plating layer tends to be uneven, and the appearance tends to deteriorate.
Japanese Patent Laid-Open No. 7-3417 Japanese Patent Laid-Open No. 4-147533 Japanese Patent Laid-Open No. 4-013855 JP-A-3-055304 Japanese Patent Laid-Open No. 10-88309 Japanese Patent Laid-Open No. 9-202952 Japanese Patent Publication No. 61-32375 JP 59-67319 A JP 59-74231 A JP-A-5-106003 Japanese Patent Laid-Open No. 10-287964 JP-A-8-269665

  The problem to be solved by the present invention is to provide an alloyed hot-dip galvanized steel sheet excellent in corrosion resistance, workability, and paintability using an ultra-low carbon steel sheet as a base plate and a method for producing the same.

  Based on the knowledge of Patent Documents 1 and 2, the present inventors have studied the alloyed hot-dip galvanized steel sheet excellent in corrosion resistance, workability, and paintability using an ultra-low carbon steel sheet as the original sheet, and completed the present invention. did. The gist is as follows.

(1) After cleaning the surface of an ultra-low carbon steel sheet containing C: 0.005% or less in mass% that has been annealed, Ni pre-plating of 0.1 to 1.0 g / m ² is performed, and a non-oxidizing or reducing atmosphere After rapid heating at a plate temperature of 430 to 500 ° C. at a heating rate of 30 ° C./sec or more, plating is performed in a hot dip Zn plating bath containing Al: 0.1 to 0.2% by mass, and wiping is performed. Corrosion resistance characterized by performing rapid heating to a temperature of 470 to 600 ° C. at a temperature rising rate of 30 ° C./sec or more and then cooling without taking a soaking time, or cooling after keeping soaking for less than 15 seconds. , A method for producing alloyed hot-dip galvanized steel sheets with excellent workability and paintability.

  ADVANTAGE OF THE INVENTION According to this invention, the alloyed hot-dip galvanized steel plate excellent in the corrosion resistance, workability, and coating property which used an ultra-low carbon steel plate as a base plate, and its manufacturing method can be provided.

Hereinafter, the best mode for carrying out the present invention will be described.
The ultra-low carbon steel sheet targeted by the present invention is one in which Ti, Nb, etc. are added alone or in combination to eliminate solid solution carbon, and further, the strength is improved by adding P, Mn, Si, etc. Etc. can be used. Moreover, what contains what is called a trump element, such as trace amount Ni, Cu, Sn, Cr, can be used. In addition, in the following description, component content shows the mass%.

  As an ultra-low carbon steel sheet in which Ti and Nb are added alone or in combination to eliminate solute carbon, in particular, by mass, C: 0.005% or less, Si: 0.03% or less, Mn: 0.05 to A material containing 0.5%, P: 0.02% or less, S: 0.02% or less, and Ti (and / or Nb): 0.001 to 0.2% can be used. Even when Ti (or Nb) is added alone, inclusion of about 0.001% or less of Nb (or Ti) mixed as an inevitable impurity is included.

  In addition, as an ultra-low carbon steel sheet whose strength is improved by adding P, in detail, C: 0.005% or less, Si: 0.03% or less, Mn: 0.05 to 0.5%, P: 0 0.02 to 0.1%, S: 0.02% or less can be used. These can be applied as an original plate of a high strength alloyed hot-dip galvanized steel sheet with good drawability, which can be applied to an automobile outer plate of 340 to 390 MPa class. Further, a composition having Mn of 0.5 to 2.5% and Si of 0.5% or less can be used. These can be applied as an original plate of a high-strength alloyed hot-dip galvanized steel sheet with good drawability, which can be applied to a 390-440 MPa class automobile outer plate.

Next, the reasons for limiting the composition and structure of the plating layer will be described.
The reason why Fe is 8 to 13% is that the corrosion resistance tends to be deteriorated if it is less than the lower limit, and the powdering property tends to be deteriorated if it exceeds the upper limit.
Ni: 0.05 to 1.0% is because the corrosion resistance is likely to be deteriorated if it is less than the lower limit, and the powdering property is likely to be deteriorated if the upper limit is exceeded. In order to obtain better powdering properties, Ni: 0.1 to 0.5% is desirable.

  The reason why Al is 0.15 to 1.5% is that powdering properties and corrosion resistance are liable to deteriorate if the amount is less than the lower limit, and coating properties and corrosion resistance are also liable to deteriorate if the upper limit is exceeded. When obtaining a better powdering property, the lower limit of Al is preferably 0.3%, and when obtaining a better paintability, the upper limit of Al is preferably 0.8%.

  Furthermore, the reason why the Al / Ni ratio is defined as 0.5 to 5.0 is that the powdering property tends to be deteriorated if the ratio is less than the lower limit, and the coating property and the corrosion resistance tend to be deteriorated if the upper limit is exceeded. When obtaining better powdering properties, it is desirable to set the lower limit of the Al / Ni ratio to 1.0.

The present invention is characterized in that the average thickness of the Γ layer at the iron-iron interface is 1 μm or less, and the variation is ± 0.3 μm or less.
Here, as a means for measuring the thickness of the Γ layer, for example, an electrolytic exfoliation method for quantifying the Γ layer by constant current electrolysis after dissolving other than the Γ layer by constant potential electrolysis in an aqueous ammonium chloride solution, Either a method of etching with a known etching solution such as nital (alcohol + nitric acid) and directly observing with an optical microscope or the like, or a method of obtaining from the X-ray diffraction intensity is possible.
Further, the variation of the Γ layer means that the maximum value and the minimum value are within ± 0.3 μm with respect to the average value of the Γ layer by measuring several to several tens of points in the width direction of the steel sheet. The upper limit of the average thickness of the Γ layer of the present invention is 1 μm, which is a relatively large value. However, the control of the above-mentioned variation is important for powdering and workability, and in addition to the above-mentioned appropriate plating composition. It is possible to obtain good performance.

Next, a method for producing the galvannealed steel sheet according to the present invention will be described.
In the present invention, an annealed ultra-low carbon steel plate is used as the original plate. First, it is necessary to clean the surface, but this method is not particularly limited, and known methods such as alkaline degreasing, brushing treatment, acid treatment, etc. are used alone or in combination depending on the state of the original plate and the state of the oxide film. Can be used. From the viewpoint of uniformity of Ni plating described later, it is preferable to use alkaline degreasing (for example, NaOH aqueous solution treatment) and acid treatment (for example, sulfuric acid aqueous solution treatment) in this order.

In the present invention, 0.1 to 1.0 g / m ² of Ni pre-plating is applied after cleaning the surface of the original plate. Although depending on the above-mentioned pretreatment for cleaning, if it is less than the lower limit, the wettability of the subsequent hot dipping is insufficient and the corrosion resistance is also insufficient. When the upper limit is exceeded, the powdering property tends to deteriorate. When seeking better powdering properties, the upper limit of Ni pre-plating is desirably 0.8 g / m ² .

  After the Ni pre-plating, rapid heating is performed at a temperature increase rate of 30 ° C./sec or more at a plate temperature of 430 to 500 ° C. in a non-oxidizing or reducing atmosphere. This treatment is necessary in order to ensure wettability of the molten plating and plating adhesion. When obtaining a better powdering property, the upper limit of the heating plate temperature is preferably 480 ° C.

  As the hot dip galvanizing bath, a bath comprising Al: 0.1 to 0.2%, inevitable impurities, and the balance Zn is used. This is because if it is less than the lower limit of Al, powdering properties and corrosion resistance are likely to deteriorate, and if it exceeds the upper limit, paintability and corrosion resistance are also likely to deteriorate. In the present invention, Ni is not positively added to the plating bath, but this point is different from Patent Documents 5 and 6, and Ni pre-plating is used as the Ni source for the plating layer. The dross is brought into the plating layer, the plating layer becomes non-uniform, and as a result, problems such as deterioration in performance do not occur. When obtaining better powdering properties, the lower limit of the bath Al concentration is preferably 0.12%.

After plating, after wiping, rapidly heat to 470-600 ° C at a heating rate of 30 ° C / sec or more, and cool without taking soaking time, or cool after holding soaking for less than 15 seconds Alloying treatment is performed. This regulation is extremely important for the suppression of the Γ layer, particularly for the variation. In particular, when the rate of temperature rise is less than 30 ° C./sec, both the Γ layer and its variation increase.
After rapid heating, it is important to cool without taking a soaking time, or to cool after a short period of time (less than 15 seconds) soaking. The layer and its variation also increase.

  In addition, it is desirable to cool a normal ultra-low carbon steel sheet without taking soaking time. Since this does not require soaking time, the furnace equipment length can be shortened, and it is not necessary to decelerate for soaking, which is advantageous in terms of productivity. Moreover, since the ultra-low carbon steel plate which added P etc. improved the intensity | strength tends to be alloyed slowly, what is necessary is just to perform soaking | uniform-heating maintenance for a short time as needed. When seeking better powdering properties, perform rapid heating to 470-550 ° C at a heating rate of 30 ° C / sec or more, and cool without taking a soaking time, or keep soaking for less than 10 seconds It is desirable to perform the alloying process by cooling after the step.

Hereinafter, the present invention will be described in detail by way of examples.
(Examples 1-13 and Comparative Examples 1-11)
Table 1 shows the components of the annealed ultra-low carbon steel sheet used in the test. After pre-treatment under the conditions shown in Table 2, Ni pre-plating was performed by electroplating (bath temperature 60 ° C., current density 30 A / dm ² ) in the plating bath shown in Table 3.
After that, it is heated to 450 ° C. in a 3% H ₂ + N ₂ atmosphere at a heating rate of 50 ° C./sec. Then, alloying was performed immediately above the wiping at a predetermined heating rate, temperature, and soaking time.
Cooling was performed by slow cooling at 2 ° C./sec for 10 sec and then rapidly cooling at 20 ° C./sec. Thereafter, 0.5% temper rolling was performed.

Samples were manufactured under various conditions shown in Table 4 (pre-Ni adhesion amount, Al concentration of plating bath, alloying conditions). The weight per unit area was 50 g / m ² .
Table 5 shows the results of measuring the composition of the plating layer and the thickness of the Γ layer in the samples of Table 4. The plating layer was dissolved in hydrochloric acid to determine the concentration of each component. Further, the Γ layer was measured at 10 points by the electrolytic stripping method, and the average value, maximum value, and minimum value were obtained. Regarding the variation of the Γ layer, any of the maximum value-average value and the average value-minimum value exceeding 0.3 μm was indicated as “x”.

Table 6 shows the performance evaluation results. The performance evaluation was performed as follows.
(1) Appearance of plating: Visual observation was conducted, and “◯” was given for those having no defects such as non-plating, “Δ” was given for some, and “X” for severe ones.
(2) Workability (powdering property): A sample coated with rust-preventive oil was subjected to a 40 mmφ cylindrical press (drawing) under the condition of a drawing ratio of 2.2, and the side surface was peeled off with tape to black The degree of conversion was evaluated. The degree of blackening of 0 to less than 20% was evaluated as “◯”, the degree of less than 20 to 30% was evaluated as “Δ”, and 30% or more was evaluated as “x”.

(3) Workability (slidability): A flat plate continuous sliding test was performed on a sample coated with rust preventive oil. The continuous sliding was performed 5 times with a crimping load of 500 kgf, and the evaluation was made with the fifth coefficient of friction. A coefficient of friction of less than 0.15 was evaluated as “◯”, a value of less than 0.15 to 0.2 as “Δ”, and a value of 0.2 or more as “x”.
(4) Corrosion resistance (coating flaw red rust resistance): Trication conversion treatment for automobiles [below note 1] and cationic electrodeposition coating [note 2] (20 μm) are applied to the steel sheet sample, then into a 5 mm × 50 mm slit shape. The coating film was peeled off to expose the plated surface, and a corrosion cycle test [Note 3] was performed. The appearance was evaluated after 10 days. No occurrence of rust or occurrence of yellow rust was evaluated as “◯”, less than 20% of red rust was evaluated as “Δ”, and 20% or more of red rust was evaluated as “×”.

(5) Corrosion resistance (perforation resistance): After smoothing a sample subjected to U-bending press with beads, a 40 mm × 40 mm mask is applied, and trication conversion treatment for automobiles [Note 1], cationic electrodeposition coating [Note 2] (20 μm) was applied. The unpainted portion from which the mask was removed with a bent plate and a flat plate was aligned with a 0.5 mm spacer so as to be inside-in to produce a vehicle body hem model. A corrosion cycle test [Note 3] was performed on this sample. The appearance after 30 days was evaluated. Less than 20% of red rust was evaluated as “◯”, 20 to less than 50% of red rust was evaluated as “Δ”, and 50% or more of red rust was evaluated as “×”.
(6) Paintability: Steel plate samples were subjected to trication conversion treatment [Note 1] and cationic electrodeposition coating [Note 2] for automobiles. Electrodeposition coating was performed under conditions of a voltage of 220 V, an upslope of 0.5 minutes, and a total current of 3 minutes, and the number of abnormalities such as craters in the test piece (70 × 150 mm) was counted. No abnormality was evaluated as “◯”, 1 to less than 3 as “Δ”, and 3 or more as “x”.
[Note 1]: Nippon Paint Co., Ltd. SD5000
[Note 2]: Nippon Paint Co., Ltd. PN120M
[Note 3]: SST (6 Hr) → Dry 50 ° C. 45% RH (3 Hr) → Wet 50 ° C. 95% RH (14 Hr) → Dry 50 ° C. 45% RH (1 Hr)
As can be seen from Tables 1 to 6 below, it was confirmed that excellent characteristics can be obtained with those within the scope of the present invention.

(Examples 14 to 22 and Comparative Examples 12 and 13)
Table 7 shows the components of the annealed ultra-low carbon steel sheet used in the test. After pre-treatment under the conditions shown in Table 2, Ni pre-plating was performed by electroplating (bath temperature 60 ° C., current density 30 A / dm ² ) in the plating bath shown in Table 3.
After that, it was heated to 455 ° C. at a temperature increase rate of 50 ° C./sec in an atmosphere of 4% H ₂ + N ₂ , immediately immersed in a molten Zn plating bath kept at 450 ° C., held for 2.5 sec, and then wiped. The basis weight was adjusted, the temperature was increased immediately above the wiping at 50 ° C./sec, and after holding for 4 sec, it was rapidly cooled at 50 ° C./sec. Thereafter, 0.5% temper rolling was performed.

(Comparative Example 14)
Table 7 shows the components of the annealed ultra-low carbon steel sheet used in the test. After pre-processing under the conditions shown in Table 2, the sample was heated to 650 ° C. at a rate of temperature increase of 20 ° C./sec in an atmosphere of 4% H ₂ + N ₂ , kept at 60 sec, and then allowed to cool to 455 ° C. After immersing in a hot-dip Zn plating bath kept at 450 ° C. and holding for 2.5 seconds, wiping is performed to adjust the basis weight, the temperature is raised directly above the wiping at 50 ° C./sec, and after holding for 4 seconds, 50 ° C./sec. It was cooled quickly. Thereafter, 0.5% temper rolling was performed.

Samples were produced under various conditions shown in Table 8 (pre-Ni adhesion amount, Al concentration of plating bath, alloying conditions). The weight per unit area was 50 g / m ² .
Table 9 shows the results of measuring the composition of the plating layer and the thickness of the Γ layer in the samples shown in Table 8. The plating layer was dissolved in hydrochloric acid to determine the concentration of each component. Further, the Γ layer was measured at 10 points by the electrolytic stripping method, and the average value, the maximum value, and the minimum value were obtained. Regarding the variation of the Γ layer, any of the maximum value-average value and the average value-minimum value exceeding 0.3 μm was indicated as “x”.

Table 10 shows the performance evaluation results. The performance evaluation was performed in the same manner as in the previous example. However, “workability (powdering)” was performed under more severe conditions (drawing ratio 2.3). The evaluation criteria are the same as in the previous example. Here, in addition to the evaluation of the previous example, low temperature chipping property was added. The low temperature chipping property was performed as follows.
After the electrodeposition coating was performed by the method of the previous evaluation item (6), the polyester-based intermediate coating material 30 μm and the top coating material 40 μm were further applied, and then left for 1 day (size is 70 mm × 150 mm). The coated sample is cooled to −20 ° C. with dry ice, and approximately 0.4 g of crushed stone (10 pieces) is vertically irradiated at an air pressure of 2 kgf / cm ² to remove the paint film that has been lifted by chipping, and then peeled off. The maximum diameter was measured. The peel diameter of less than 4 mm was evaluated as “◯”, 4 mm to less than 6 mm as “Δ”, and 6 mm or more as “x”.

  As described above, it was confirmed that excellent characteristics can be obtained in the scope of the present invention.

According to the present invention, an alloyed hot-dip galvanized steel sheet having excellent corrosion resistance, workability, and paintability is obtained using an ultra-low carbon steel sheet mainly used for automobiles as a base plate, and its industrial utility value is great. is there.

Claims (1)

0.1% to 1.0 g / m after cleaning the surface of the ultra-low carbon steel sheet containing C: 0.005% or less in mass% annealed ² Ni pre-plating,
After rapid heating in a non-oxidizing or reducing atmosphere at a plate temperature of 430 to 500 ° C. at a heating rate of 30 ° C./sec or more, a molten Zn plating bath containing Al: 0.1 to 0.2% by mass Plated inside,
After wiping, rapid heating is performed at 470 to 600 ° C. at a heating rate of 30 ° C./sec or more, and cooling is performed without taking a soaking time, or cooling is performed after holding the soaking for less than 15 seconds. A method for producing galvannealed steel sheets with excellent corrosion resistance, workability, and paintability.